Lipids

In this lesson, you're going to learn about the different types of lipids, and their structure and function in your body. Specifically, you will look at the following:

1. Lipid Characteristics Overview
2. Fats
3. Triglycerides
4. Phospholipids
5. Sterols

1. Lipid Characteristics Overview

Lipids are molecules that are nonpolar. They contain hydrogen, carbon, and oxygen. They are an organic molecule, and they don't dissolve in water, which goes along with them being nonpolar.

Lipid

An organic molecule made up of glycerol and fatty acids, which is non-polar and does not dissolve easily in water

Nonpolar

Does not dissolve easily in water

2. Fats

Fats are a type of lipid that have a fatty acid tail attached to a glycerol molecule.

Fat

A lipid made of a glycerol molecule and a fatty acid side chain

Each of those fatty acid tails contains up to 36 carbons as the backbone of the fatty acid, and also contains a carboxyl group at one end. Most of the bonding space that's available in the fatty acid tail is occupied by hydrogens.

There are two main categories of fats.

1. Saturated fats

Saturated fats are a type of fat that only contain single covalent bonds in the fatty acid tail.

Saturated Fat

A type of fat which does not have double bonds in its side chain

Oftentimes saturated fats are animal fats and they're generally solid at room temperature, because they're more tightly packed together.

An example of a saturated fat would be lard.

2. Unsaturated fats

Unsaturated fats are a type of fat that has one or more double covalent bonds between carbons, which are the backbone of the fatty acid.

Unsaturated Fat

A type of fat which has double bonds in its side chain

An example of an unsaturated fat might be vegetable oil or olive oil.

Unsaturated fats are generally more plant-based, whereas saturated fats are more animal-based. Unsaturated fats are thus less tightly packed, and are usually liquid at room temperature.

Unsaturated fats are also generally a little bit healthier than saturated fats. However, there are some unsaturated fats, such as trans fats, that are more unhealthy.

Trans Fat

A type of unsaturated fat which is unhealthy and may lead to health problems

To take a look at what saturated fats and unsaturated fats might look like, see the glycerol molecule below. This is an example of a saturated fat.

The fatty acid tail is attached to a glycerol molecule. You have a glycerol molecule on the top, and then a fatty acid tail connected to that. The fatty acid tail is made of a carbon backbone, and it can have up to 36 of those. That will vary depending on the type of fat that it is, but you will have the carbon bonded to hydrogens.

You'll notice that they are all single covalent bonds. There aren’t any double bonds between hydrogens in this example. You'll also notice that they're all single bonds between the carbons, while most of the bonding space is occupied by hydrogens. 

Now see the unsaturated fat image below.

Unsaturated fats have one or more double covalent bonds between the carbons within your carbon backbone. You'll notice the double bond between the carbons makes this an unsaturated fat.

In unsaturated fats, because of the double bonds, the fatty acid tail is usually kinked, which, again, allows them to be packed less tightly, making them generally liquid at room temperature.

Another type of unsaturated fat is something called a polyunsaturated fat. The prefix "poly" means many; this type of unsaturated fat could have multiple double bonds.

3. Triglycerides

Triglycerides look similar in structure to fats, but the prefix "tri" means three, so rather than just having one fatty acid tail, they have three. Triglycerides are the most common lipid in your body, and they contain lots of energy. An example of a triglyceride might be butter, lard, or oils.

Triglyceride

A type of lipid which is made up of a glycerol molecule and three fatty acid side chains

4. Phospholipids

Phospholipids are a type of lipid found in the cell membrane of your body's cells. Phospholipids are made up of a hydrophilic head and two hydrophobic tails.

Phospholipid

A type of lipid made up of a hydrophilic head and hydrophobic tail, and is found in cell membranes

Something that is hydrophilic is attracted to water, whereas something that is hydrophobic is repelled by water. So in the structure of your cell membranes, the phospholipids are arranged in a bilayer.

See the drawing of a phospholipid below.

The head faces out, and all of the tails face in. The heads are hydrophilic, meaning they're attracted to water. So those heads are pointed to the outside, or to the inside of the cell, where there's water. The hydrophobic tails are pointed inward, away from water.

5. Sterols

Sterols are lipids that don't have a fatty acid tail so their structure is a little bit different.

Sterol

A type of lipid containing only a glycerol molecule

One common example of sterols would be cholesterol in your body. It's a lipid that doesn't have a fatty acid tail. Steroid hormones, like estrogen and testosterone, are also examples of sterols.

This lesson has been an overview of the different types of lipids, and the structure and function of them in your body. In this lesson, you learned that lipids are nonpolar, and don't dissolve in water. They are organic molecules containing hydrogen, carbon, and oxygen. You also went more in-depth about fats, triglycerides, phospholipids, and sterols.

Keep up the learning and have a great day!